Comfort and road handling performance of a passenger car or any other vehicle are mainly determined by the damping characteristic of the shock absorbers on the vehicle. Passive shock absorbers have a fixed damping characteristic determined by their design. Depending on the road excitation, however, it is desirable to adjust this characteristic to increase performance. Semi-active and active suspension systems offer the possibility to vary the damper characteristics along with the road profile by, for example, changing the restriction of one or two current controlled valves or by changing the viscosity of a magneto rheological fluid.
An active shock absorber has the additional advantage that negative damping can be provided and that a larger range of forces can be generated at low velocities, thereby potentially allowing an increase in system performance. However, semi-active suspensions are less complex, more reliable and more commercially available than active suspensions. They do not require an external power source (e.g., a hydraulic pump) and are more safe because they can only dissipate energy and therefore cannot render the system unstable.
There exist several linear and nonlinear methods to control a car using an active or semi-active suspension. As for the known linear methods, they generally apply linear control strategies based on linear physical car models consisting of lumped masses, linear springs and dampers, and a shock absorber modeled as an ideal force source. However, real car dynamics are much more complex and active shock absorbers are not ideal force sources but have a complex nonlinear dynamic behavior. The unrealistic assumptions used by the known methods make these linear control approaches less appropriate for practical applications.
Nonlinear control methods such as linear parameter varying gain scheduling, backstepping, and adaptive control have been applied to active suspension systems. These controllers are based on a nonlinear physical car and damper model which have a large number of parameters. The experimental identification of these model parameters is a complex problem. In addition, the design and tuning of a nonlinear controller using these known methods is difficult, and therefore the use of nonlinear models and controllers lead to very time-consuming designs, since no standard techniques or software tools are available.
Lauwerys et al., “Design and experimental validation of a linear robust controller for an active suspension of a quarter car”, Proceeding of the American Control Conference (2004), discloses a practical, experimental approach using linear identification and robust control techniques on an active suspension of a quarter car test rig. A linear robustly performing controller is obtained using μ-synthesis based on an experimentally identified linear model of both the active suspension and the quarter car dynamics. The relatively simple construction of the test rig and the linearity of the active suspension made it possible to apply linear identification and control design techniques. However, the dynamics of a real car are much more complex and a semi-active suspension behaves quite differently then an active suspension because, for example, it becomes uncontrollable when the rattle velocity is zero.
The above-described model based methods may, in theory, yield optimal controllers for certain shock absorbers and car models. However, their application to a full car and highly nonlinear semi-active shock-absorbers is complex and very difficult, if not impossible, to implement.
Based on the foregoing, there is a need for a system and method for model free control of a nonlinear semi-active or active shock absorber.